1. Field of Invention
This invention relates generally to screw-type masonry anchors for fastening an attachment to masonry, the anchor being turned into a hole drilled in the masonry to mechanically retain the anchor therein, and more particularly to an anchor of this type having one or more cutting threads which enhance the holding power of the anchor. This invention also relates to a sleeve anchor for use when such masonry bolts are used with cinder blocks or other masonry having thin-walled areas.
2. Status of Prior Art
In order to fasten fixtures and other attachments to masonry, it is known for this purpose to use a screw-type anchor, such as the anchors disclosed in the Giannuzzi U.S. Pat. Nos. 5,118,496 and 5,282,708. In anchors of this type, a helical cutting thread is formed on the shank of the anchor which acts to cut and tap the bank of a hole drilled in the masonry when the anchor is turned into this hole; thereby mechanically retaining the anchor therein. The resistance of this anchor to axial pull-out forces represents its holding power.
The term masonry is generic to all stone-like building materials, such as concrete and brick. As pointed out in British patent application GB 2115511 A to Godsted, when masonry is exceptionally hard, such as masonry whose aggregate is formed of granite, then in order to turn a screw-type masonry anchor into a hole drilled therein, the torque required for this purpose is very high. The torque is so high, in fact, that when seeking to screw an anchor into the masonry hole, the anchor could then break.
To create an anchor capable of being screwed into a hole drilled in exceptionally hard masonry, Godsted""s anchor is provided with a helical thread having a V-shaped cross sectional configuration in which the flanks of the thread intersect at an included angle of 50 to 65 degrees.
The screw-type masonry anchor disclosed in the Ernst U.S. Pat No. 3,937,119 has a sharp-crested helical thread in which notches are formed in the crests of the thread to facilitate embedment of the crests in the bank of the drilled masonry hole. Aggregate particles cut from the bank of the masonry hole by the notches in the crest are collected in a reservoir in the space between successive convolutions of the thread. According to Ernst, because the particles are collected, they do not produce torsional friction and make it possible to turn the anchor into the hole at relatively low torque levels. Also according to Ernst, the retention of these particles in the reservoirs xe2x80x9cwould enhance the pull out force of the anchor.xe2x80x9d But Ernst does not explain how loose particles in this reservoir act to enhance the resistance of the anchor to axial pull out forces.
Bickford, in U.S. Pat No. 5,531,553, discloses a self-tapping masonry anchor having a helical cutting thread surrounding the shank of the anchor. This cutting thread is formed by a pair of parallel helical ridges having a depressed groove therebetween serving to collect substrate debris removed by the cutting action. To facilitate self-tapping, the helical and separating the successive convolutions of the parallel ridges has a width measured axially with respect to the shank of at least 4 mm and at least 50% of the land shank diameter. The helix angle of the helical ridge in Bickford is at least 10%. In Bickford, even if particulate debris collected in the groove in the helical cutting thread were compacted therein, it would only serve to resist displacement of the helical thread with respect to the corresponding female thread cut into the bank of the hole; but it would not serve to significantly enhance the holding power of the anchor. Also in Bickford whose cutting thread has two parallel ridges, these ridges are in close proximity to each other, with a broad land between successive convolutions of the thread. The propinquity of the ridges does not allow each individual ridge to utilize the maximum shear strength available in the masonry material.
Of potentially greater interest is Tajima, U.S. Pat. No. 4,652,194, in which particulate debris produced by the cutting action of a screw-type anchor serves to enhance the holding power of the anchor. Tajima points out that prior to his invention it was known to exploit the space between successive convolutions of the thread to collect and compact therein debris produced by the cutting thread to increase the pull-out resistance of the anchor. According to Tajima, the spaces in prior art anchors were unduly large and it was therefore not possible for these spaces between successive convolution to be completely filled up and tightly compacted with particles to wedge the anchor in the hole. To overcome this drawback, Tajima provides a triangular indentation in the land between successive convolutions of his helical thread. This indentation is filled with particles scraped off the bank of the masonry hole by the cutting action of the thread. However, an indented land creates a relatively large space and the particles in the space may not become tightly compacted to wedge the anchor in the hole.
In Giannuzzi et al., U.S. Pat. No. 5,957,646, an anchor is provided with a shank having a root section surrounded by at least one helical male cutting thread whose crest diameter exceeds the diameter of the hole whereby when the anchor is screwed into the hole, the cutting thread then cuts a female thread in the bank of the hole to mechanically retain the anchor therein to impart to the anchor a holding power that resists axial pull-out forces. Formed on the root in the helical land between successive convolutions of the thread is a ridge that is raised above the root to define a compacting zone between the ridge and the bank of the hole. Particulate debris produced by the cutting action is discharged into the land and forced into the compacting zone to create a dense mass that wedges the anchor in the hole and thereby enhances the holding power of the anchor.
While these are all innovative designs, it would be beneficial to provide a bolt having both additional wedging action as well as additional holding action.
In another aspect, none of these devices is directed to the problem of using a masonry bolt on a cinder block or other hollow or thin-walled ceramic or masonry block. Such blocks typically have a thin wall of a thickness less than the length of any masonry bolt to be inserted. Thus, when a hole is drilled and a masonry bolt inserted, the bolt does not grab the wall well and even if it cannot be pulled out, usually it can be unscrewed from the wall with very little backout torque (e.g., if Giannuzzi""s bolt is unscrewed xc2xc turn with a wrench, it can thereafter be unscrewed with one""s fingers, similar to a wood screw).
In view of the foregoing, one object of this invention is to provide a self-tapping, screw-type anchor for fastening an attachment to masonry, the anchor being screwed into a hole drilled in the masonry and being mechanically retained therein to resist both pull-out and unscrewing forces, the holding strength and resistance to pull-out and unscrewing being enhanced by compacted particulate debris that creates a mass that wedges the anchor in the hole.
More particularly, an object of this invention is to provide an anchor of the above type in which the anchor is mechanically retained in the hole by multiple lead male cutting threads surrounding the shank of the anchor which are separated from a nub by low-lying lands, and in which particulate debris produced by the cutting threads is compacted by the nub to create a dense mass that wedges the anchor in the hole.
Briefly stated, these objects are attained by a screw-type, self-tapping masonry anchor for fastening a fixture or other attachment to masonry in which a hole is drilled to receive the anchor. The anchor is provided with a shank having a root section surrounded by one or two helical male cutting threads whose crest diameters exceed the diameter of the hole whereby when the anchor is screwed into the hole, the cutting threads then cut corresponding female threads into the bank of the hole to mechanically retain the anchor therein to impart to the anchor a holding strength that resist axial pull-out forces. The threads in cross section have a trailing face that is essentially normal to the shank and a leading face that is essentially radial in geometry. Formed on the root in the helical land between successive convolutions of the thread is a hemispherical ridge that is raised above the root of the thread and provides a diameter larger than the hole into which the anchor is inserted but less than the diameter of the crest of the cutting threads.
In summary, this invention provides a screw-type self-tapping masonry anchor bolt for fastening an attachment to friable masonry having a hole drilled therein to receive the bolt, the hole having a predetermined diameter, the bolt comprising a shank having a root section whose diameter is smaller than that of the hole and a shank being integral with a head adapted to be engaged by a torque-producing tool, at least one helical male cutting thread surrounding the root and having a crest diameter exceeding that of the hole whereby when the head is engaged by the tool and the bolt is screwed into the hole the male cutting thread then-cuts a female thread into the bank of the hole to mechanically retain the anchor and resist pull-out forces to impart holding strength to the anchor, the convolutions of the thread having a helical land or space therebetween being less than the predetermined diameter, and a hemispherical ridge formed in the helical land or space and having a diameter greater than the predetermined diameter and less than that of the crest diameter effective to wedge into the bank of the hole.
While not desirous of being constrained to any particular operating principle, it is believed that the cutting tooth acts to oppose pull-out forces, while the ridge provides a wedging action along its entire helical extent the opposes unscrewing forces. The helical nature of the cutting thread provides a mechanical advance allowing the frictional forces of the wedging ridge to be overcome when inserting the bolt by screwing.
Another object of this invention is to provide a device and method for using such an anchor in a cinder block or other thin-walled masonry block, and especially to prevent such an anchor from being unscrewed therefrom.
In summary, this invention provides a sleeve used for inserting a masonry anchor into a hole having a predetermined diameter in a thin-walled masonry block, the sleeve comprising a deformable polymeric material having an inner diameter approximately equal to the diameter of root of the masonry anchor and an outer diameter approximately equal to the predetermined diameter of the hole.